The present invention relates to a push button switch covering assembly or, more particularly, to a push button switch covering assembly consisting of a push button switch covering member and a clicking member therebelow to impart the operator's finger tip with a sharp and pleasant feeling of clicking despite a very small switching stroke when the push button of the switch is pushed down to close the electric circuit. Such a push button switch covering assembly is used in many electric and electronic instruments such as pocketable calculators, remote controllers of electric and electronic appliances, telephone panels and so on for inputting operation signals to the instrument.
FIG. 7 of the accompanying drawing illustrates a typical example of the push button switching unit of the prior art consisting of a push button switch covering member 21 and a base board 28 for signal inputting by a vertical cross sectional view. The push button switch covering member 21 is shaped integrally from a rubbery material such as a silicone rubber. The push button switch covering member 21 is an integral body consisting of a key top 22, a base plate 24 and a thin-walled riser part 23 connecting the key top 22 and the base plate 24 and resiliently deformable when the key top 22 is pushed down by the finger tip of an operator so as to bring the movable contact point 25 provided on the lower surface of a downwardly raised part 27 of the key top 21 into contact with a pair of fixed contact points 29 on the base board 28 of the switching unit thus to close the electric circuit between the fixed contact points 29. It is desirable in this assembly that the elastic deformation of the riser part 23 takes place with clicking caused by buckling so that the operator's finger tip receives a definite touch feeling of switching. It is usual that the top surface of the key top 22 is provided with a layer 26 formed by printing or attaching a printed seal or label bearing indicia such as numerical figures, signs and patterns.
One of the problems in the above described push button switch covering member 21 made from a rubbery material such as a silicone rubber is that, because the rubbery material has high flexibility as compared with conventional plastic resins, the phenomenon of clicking caused by buckling takes place only with a sufficiently large stroke of pushing which is possible only by increasing the distance between the key top 22 and the base board 28 of the switching unit and, even if the phenomenon of clicking can take place, the feeling imparted to the operator's finger tip is soft and rather obscure. Needless to say, the above mentioned large distance between the key top 22 and the base board 28 is contrary to the requirement in the modern electronic instruments designed for portability that the push button switching unit must have a thickness as small as possible.
FIG. 8 of the accompanying drawing illustrates a modified embodiment of the prior art push button switch covering assembly, in which the pushing-down stroke of the key top 22 is relatively small, by a vertical cross sectional view in which the riser part 23 of the push button covering member 21 is no longer responsible for occurrence of the phenomenon of clicking by buckling in pushing down of the key top 22. Instead, a clicking member 32, which is made from a relatively rigid plastic resin such as polyethylene terephthalate and polyethylene terephthalate, having a clicking diaphragm 30 in a downwardly concave configuration, is provided below the key top 22 in contact with the downwardly protruded presser 33. The clicking member 32 and the push button switch covering member 21 are usually bonded together by adhesion. A movable contact point 25 is formed, for example, by printing with an electroconductive printing ink containing carbon particles on the lower surface of the clicking diaphragm 30 just to oppose the presser 33. When the key top 22 is pushed down with an operator's finger tip, the clicking diaphragm 30 is also pushed down through the presser 33 at the top thereof so that the fixed contact point 25 on the lower surface of the clicking diaphragm 30 is brought into contact with the fixed contact points on the base board (not shown in FIG. 8). In this case, buckling deformation of the clicking diaphragm 30 takes place with resilience even when the pushing stroke is as small as to be 0.5 to 0.7 mm thus to impart a sharp and definite feeling of clicking to the operator's finger tip.
A problem in the push button switch covering assembly of each of the embodiments illustrated in FIGS. 7 and 8 is that, since the key top 22 of the push button switch covering member 21 is provided on the lower surface with a downward protrusion 27 or downwardly extended presser 33, the lower surface of the key top 22 is not available for printing of indicia but the layer of indicia 26 must be provided always on the upper surface of the key top 22. Needless to say, such a design of the layer of indicia 26 on the upper surface of the key top 22 is disadvantageous because the indicia are subject to wearing or fading away in the lapse of time due to rubbing or repeated pushing with an operator's finger tip.
As a solution of the above mentioned problem due to wearing or fading away of the indicia formed on the upper surface of the key top 22 of the push button switch covering member 21, a modification of the embodiment illustrated in FIG. 8 is proposed in which, as is illustrated in FIG. 9 by a vertical cross sectional view, the indicia layer 26 on the upper surface of the key top 22 is protected by adhesively bonding thereto a key top protector 22A made from a transparent material. In this embodiment, the indicia layer 26 is of course safe from wearing because the layer 26 is not exposed but sandwiched between the key top 22 and the key top protector 22A. The embodiment illustrated in FIG. 10 is a further modification of that of FIG. 9, in which the key top 22 is in the form of a readily deformable membrane with omission of the riser part 23 shown in FIG. 8. The embodiment illustrated in each of FIGS. 9 and 10, however, is disadvantageous because the key top protector 22A of a transparent material must be prepared separately from the push button switch covering member 21 and the separately prepared pieces of the key top protector 22A must be adhesively bonded each to one of the key tops 22 one by one with exact positioning resulting in a great increase in the production costs of the push button switch covering assemblies.